Specialist predators with a limited diet may be less adaptable to environmental change than generalists, which consume a diversity of prey. As the climate changes, ecological homogenization is occurring, where generalist species outcompete specialists, reducing ecosystem complexity. In Arctic ecosystems, temperatures are rising at almost twice the rate of the rest of the planet and summer rainfall has increased significantly over the last century. Arctic peregrine falcons (Falco peregrinus. tundrius) breeding throughout the circumpolar regions of Canada, the USA, and Greenland have a diversity of prey species to choose from, but little is known about how they select for these resources as prey populations fluctuate. To fully appreciate the potential implication of climate change on Arctic peregrine falcon populations, research investigating prey selection and the potential for peregrines to switch to alternative prey is needed. In this dissertation I studied an Arctic tundra ecosystem around Igloolik, Nunavut, and related spatiotemporal variations in all prey species consumed by peregrines to annual variation in weather, while simultaneously monitoring peregrine diet and reproductive output. First, I investigated the relationship between annual variation in weather and spatiotemporal variation in the abundance of multiple avian guilds: songbirds (Passeriformes), shorebirds (Scolopacidae and Charadriidae), gulls (Laridae and Sternidae), loons (Gaviiformes), geese and ducks (Anatidae), and black guillemots (Cepphus grylle). I spatially stratified my study area and conducted distance sampling to estimate strata-specific densities of each guild during the summers of 2010-2012 while also monitoring temperature and rainfall. Shorebirds, songbirds and gulls were less abundant in 2012, which was a cool and wet summer, relative to 2010 and 2011. I monitored annual variation in lemming abundance using snap trapping and assumed spatial variation in lemming density was proportional to spatial variation in the density of lemming burrows observed while surveying distance sampling transects. Lemming density was at least 25 times higher in 2011 than in 2010 and 2012. I quantified peregrine nestling diet using a combination of two methods: direct observations from motion-sensitive cameras at nest sites and stable isotope analysis. I developed a novel method of incorporating unidentified food items from nest camera images into diet estimates and quantifying error around these estimates for individual nests, which were used as informative prior hypotheses in Bayesian mixing models predicting nestling diets. I used stable isotope signatures of falcon nestling plasma and prey tissues samples as inputs for Bayesian mixing models. When informative priors were included, the resulting diet estimates from mixing models had narrow credible intervals and generally reflected the prior hypotheses. Nestling diets were dominated by songbirds and shorebirds (insectivorous birds), which generally contributed > 80% of total diet. The use of ducks and marine prey (gulls and black guillemots) was somewhat limited, but these prey contributed up to 50% of nestlings’ diet in nests with more access to marine habitat. Lemmings were generally not used while they were scarce in 2010 and 2012, but they contributed 20-50% of nestlings’ diet during the lemming peak in 2012. Next, I investigated whether peregrines switched to alternative prey when their primary prey of insectivorous birds declined by examining the functional response of falcons to changes in prey density. I also compared peregrine nestling survival across all years of the study to determine if reproductive output was influenced by prey density and prey intake rates. Peregrine falcons exhibited a functional response to lemmings, ducks and total prey density: intake rates decreased when prey densities declined in 2012. The number of fledglings produced per nest also declined in 2012, indicating there was a fitness consequence of declining prey densities and intake rates. Finally, I compared the proportional availability of each prey type to its proportional contribution to peregrine falcon diets and calculated selection ratios for each prey type. Peregrine falcons selected for insectivorous birds and avoided lemmings. There was no evidence that peregrines switched to alternative prey when insectivorous birds declined. Results from my research indicated that Arctic peregrine falcons specialize on insectivorous birds and use alternative prey only opportunistically to supplement their diet.

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